I am using a criteria with a custom key. That works fine when passing an ID, e.g. when using "People who bought this, bought that".
However, it is not working when passing multiple IDs, whatever the format is. Are the recommendation logics not able to work with more than one ID? Or am I doing something wrong with the formatting of the custom key?
Recommendations only supports a single key per request for item based algo types (e.g., viewed this, viewed that; viewed this, bought that; bought this, bought that). However, it has a keyless type that is meant to be based on a visitor's set of behaviors (i.e., user-based recommendations).
You could in theory make multiple requests (each with a single key) to Target to achieve a multi-key recommendation scenario and then use some client-side JS to merge the results.